1. Field of the Invention
The present invention relates to the field of polymers, and more particularly, to polymer dielectrics for use in electronic applications.
2. Background Information
There are a wide variety of polymer materials which may be used as dielectric layers and for other purposes in electronic and other applications. Each polymer composition has a set of physical properties such as density, water absorption tendency, dielectric constant, melting point or softening point, color, susceptibility to laser ablation, and so forth. A high density interconnect (HDI) system is disclosed in U.S. Pat. No. 4,783,695 to C. W. Eichelberger et al. Methods of fabricating such high density interconnect structures are disclosed in U.S. Pat. Nos. 4,714,516 and 4,835,704 to C. W. Eichelberger et al. This high density interconnect structure comprises integrated circuit chips having contact pads thereon mounted on a substrate with a dielectric layer adhesive bonded thereover. Via holes are formed through the dielectric layers and a patterned metallization layer is disposed on top of the dielectric layer and extends into the via holes to make electrical contact to the contact pads of the circuit chips.
This structure places special requirements on the dielectric materials. In particular, in order for the final structure to be usable over a wide temperature range, the dielectric layers must have high melting points and high thermal stability. They must also be laser ablatable by ultraviolet light in order to form the via holes through which different layers of metallization are connected. In the HDI system, laser processing (ablation, photoresist exposure, etc.) is normally done with one, or at most, two passes of the laser beam with a power ranging from 0.5 to 2.0 watts with a preferred maximum power level being about 1.5 watts. Thus, when a dielectric layer is characterized as being laser ablatable, it means that such a layer can be totally removed by one or two passes of a laser beam of this power level and when it is characterized as not being laser ablatable, it means that a layer is not completely removed by one or two passes of such a laser beam.
To minimize the complexity and cost of equipment for fabricating such high density interconnect structures, it is considered desired to be able to do all laser processing at a single frequency in order that only a single laser is required. Accordingly, preferred materials are those which may be processed at a laser frequency of 351 nm. This frequency was selected in accordance with the characteristics of desirable dielectric layers such as Kapton.RTM. polyimide available from E. I. DuPont de Nemours and the fact that there are commercial photoresists which can be processed at this frequency. ULTEM.RTM. polyetherimide resin available from General Electric Company has been used as an adhesive layer in this high density interconnect structure for bonding Kapton.RTM. to the underlying structures. The ULTEM.RTM. resin is laser ablatable at 351 nm. The ULTEM.RTM. material has a melting point in the neighborhood of 220.degree. C. or higher, depending on its specific formulation. This ULTEM.RTM. high temperature adhesive layer is suitable for use in permanent structures.
A need has developed for temporary structures in which it is desirable to have an adhesive layer whose melting point is in the vicinity of 150.degree. C. rather than 220.degree. C. Unfortunately, Applicants have been unable to find a suitable dielectric material having a melting point in the vicinity of 150.degree. C. which has all the other properties required for use in the HDI system. In particular, dielectric materials which are otherwise suitable are not sufficiently laser ablatable at 351 nm to be practical in the HDI system.
U.S. Pat. No. 4,617,085 to Cole, Jr. et al. entitled "Process for Removing Organic Material in a Patterned Manner from an Organic Film" discloses a combination of two different laser ablatable materials such as a blend of polymethylmethacrylate (PMMA) and poly(.alpha.-methyl styrene) (PS). While each of these materials is laser ablatable, a blend comprised of appropriate ratios of these materials is substantially more laser ablatable than either one alone.